South Atlantic Ocean, about half way between southern Africa and South America

Tristan da Cunha is the most isolated inhabited island in the world, located 2,778 km to the west of the nearest mainland of Cape Town, South Africa. The island has a unique and fascinating history. Tristan’s neighbor, Gough Island, is the least disturbed major cool-temperate island ecosystem in the South Atlantic Ocean. Both islands are British possessions, and both are very important for their endemic landbirds and seabird colonies. Large populations of fur seals and elephant seals are also residents of these islands. The undisturbed nature of Gough makes it particularly valuable for biological research.

Scientific Code

(AT0803)

Ecoregion Category

Afrotropical

Size

100 square miles

Status

Critical/Endangered

Habitats

Description Location and General DescriptionThe Tristan da Cunha Island Group (37° 06' S, 12° 18' W) is an archipelago of five volcanic islands resting on the east slope of the mid-Atlantic ridge, midway between Africa and South America. Tristan Island is the youngest island in the group, at one million years, and is still considered volcanically active with its most recent eruption occurring in 1961-62. The island is 41 km2 and has a volcanic cone that reaches 2,062 m with a base circumference of 34 km. The volcano is often snow-capped and has a crater lake. The coast is characterized by sea cliffs of up to 600 m in height, occasionally broken by low coastal strips. Above the cliffs, a 600-900 m plateau of sorts surrounds the base of the volcano. The island’s small settlement, called Edinburgh, is on the northwest coast. Other islands in the group, Nightingale Island and Inaccessible Island, are eroded volcanic cones once similar in size to Tristan Island (Ashworth et al. 2000). Two small islands near Nightingale are named Stoltenhoff Island and Center, or Middle, Island.

Often referred to as the "Remotest Island in the World", Tristan Island has an unusual human history. The island group was discovered by Portuguese Admiral Tristado d'Ancunha in 1506 (Ashworth et al. 2000), and was visited by sealers and whalers during the 18th century. The British formally annexed the islands, and a small community was founded in the early 1800’s. The island’s population had reached almost 300 in 1961, when a volcanic eruption forced the islanders to evacuate. After a period in England, the majority of the islanders chose to return to Tristan in 1963. The island’s current population has a remarkable continuity with its founding members. Tristan Islanders speak a distinct dialect of English that reflects their origins in Georgian England but is also laced with a few early Americanisms.

Gough Island (40°21'S, 09°53'W) lies approximately 425 km southeast of Tristan da Cunha, consisting of one main island and several offshore islets and rocks. Most of these are within 100 m of the main island, and the largest support vascular plants and breeding birds (WCMC 1994). Volcanic in origin, the Gough Island group covers 65 km² and has a high point of 910 m, at Edinburgh Peak. The islands’ most recent volcanic activity occurred about 2,400 years ago (Ashworth et al. 2000); Gough was part of the same volcanic mass as the Tristan formations. Gough Island is composed of a central plateau with several mountain peaks, and the coastline is marked by cliffs of 300 m to 450 m in height with narrow boulder beaches and no sheltered harbor; the only area below 200 m altitude is at the southern end of the island (WCMC 1994).

Tristan da Cunha and Gough Island are included in the Dependencies of Saint Helena, which is 2,333 km to the north. These islands have a cool-temperate oceanic climate, with mean temperatures at near sea level of 11.3°-14.5°C with little seasonal variation, though rapid weather changes are common. Snow may fall on the peaks between May and January, but rarely occurs at sea level. The islands lie on the edge of the "roaring forties", a seasonally oscillating wind belt south of 40°S (WCMC 1994). With a mean of 3,397 mm annually, Gough Island receives much more precipitation than Tristan, which averages 1,676 mm (UNEP 1989-1990).

The vegetation of the Tristan da Cunha and Gough Island group is typical of southern cold-temperate oceanic islands, having relatively low species diversity, and a large prevalence of ferns and other cryptogams. The islands support five native vegetation types zoned in relation to altitude and topography. Coastal areas are dominated by tussock grassland (Spartina arundinacea and Parodiochloa flabellata) that is restricted to areas subjected to regular salt spray (WCMC 1994). Tussock grass has largely been eliminated on Tristan by grazing animals, though it does persist on the eastern coast of the island and on inaccessible cliffs. Beyond the tussock grass, fern bush (Histiopteris incisa and Blechnum palmiforme) interspersed with an occasional "island tree" (Phylica arborea) occurs from about 300 m to 500 m elevation. Wet heath extends as high as 800 m presenting a diverse vegetation type, composed of fern species, sedges, grasses, angiosperms, and mosses. B. palmiforme, Empetrum rumbrum, and grasses and sedges dominate these heaths, respectively. Peat bogs of Sphagnum moss occupy suitable depressions above 600 m. Tetroncium magellanicium and Scirpus spp. are the only abundant vascular plants found in the bogs. From 600 m and above, feldmark and montane rock communities are found. These are composed of an assemblage of "cushion-forming" or crevice plants, growing on exposed areas such as ridges (WCMC 1994). The lower altitude islands show fewer vegetation types; for example, Inaccessible Island has three vegetation types: tussock grass, fern bush and a few freshwater bogs, and Nightingale is predominately covered by dense tussock grass. Over sixty plant taxa are restricted to the Gough and Tristan group of islands (WCMC 1994). Gough Island has only four widespread exotic plants; more occur on Tristan Island.

Biodiversity FeaturesGough Island is one of the most important seabird colonies in the world. About 20 species of seabirds breed on the island (Stattersfield et al. 1998). The Tristan Island group supports many of the same bird species as well as claiming a few unique populations. BirdLife International has classified both Tristan da Cunha and Gough as important Endemic Bird Areas. Tristan is quite remarkable in supporting three endemic genera: Atlantisia, Nesocichla, and Nesospiza (Stattersfield et al. 1998). Nesospiza buntings have speciated at the Tristan da Cunha archipelago, resulting in the Tristan bunting (Nesospiza acunhae) and Wilkins' bunting (N. wilkinsi), which are significantly different in size and co-occur without interbreeding on Nightingale Island (Ryan et al. 1994). Two endemic landbirds are found at Gough, the Gough moorhen (Gallinula comeri), which is found in fern bush vegetation areas and has also been introduced on Tristan, and the Gough finch (Rowettia goughensis). Tristan albatross (Diomedea exulans dabbenena), the most genetically distinct of the five subspecies of wandering albatross, is listed as Endangered and breeds only on Gough and Inaccessible Islands (Ryan et al. 2001). Several other birds are restricted to Tristan and Gough Islands when breeding including white-chinned petrel (Procellaria aequinoctialis conspicillata) (likely an endemic species), great shearwater (Puffinus gravis), and Atlantic petrel (Pterodroma incerta) (Stattersfield et al. 1998). Tristan da Cunha and Gough Islands are the main southern ocean breeding sites of little shearwater (Puffinus assimilis), and Gough is also a primary breeding site of Puffinus gravis, with as many as three million pairs breeding on the island (WCMC 1994). The last survivors of the southern giant petrel (Macronectes gigantous) also breed on Gough, with an estimated 100-150 pairs (WCMC 1994). Other notable birds of this island group are the endemic Inaccessible Island flightless rail (Atlantisia rogersi), the world's smallest flightless bird (Fraser et al. 1992), and the northern rockhopper penguin (Eudyptes chrysocome moseleyi), for which about 48% of the world's population breed at Gough (WCMC 1994).

Only two native breeding mammals occur on the Tristan-Gough Islands. Subantarctic fur seal (Arctocephalus tropicalis) are abundant, and southern elephant seal populations (Mirounga leonina) have been increasing. Populations of these as well as local marine mammals, southern right whale (Eubalaena glacialis australis) (EN) and dusky dolphin (Lagenorhynchus obscurus), have fluctuated greatly in recent history due to hunting.

Reptiles, amphibians, freshwater fish, and native terrestrial mammals are absent from these islands and terrestrial invertebrates have been poorly studied. In addition to the crayfish industry that largely supports Tristan Island’s economy, Tristan rock lobster (Jasus tristani) and octopus are both are economically exploited, with regulations (WCMC 1994).

Current StatusTristan Island, being the only permanently inhabited island in this group, has experienced the greatest degree of environmental degradation. Pressures have resulted from agriculture, overgrazing by sheep, tree removal, fire, and the introduction of exotic species. The exploitation of natural resources on Tristan was critical for human survival until the middle of this century, and some native species were almost or completely exterminated. The small community on the island has since taken several locally based measures to reverse this trend. The Island Council passed the first Protection Ordinance in 1950, and in 1994, Inaccessible Island was declared a nature reserve. This island is globally important for its endemic terrestrial species, including the flightless rail, as well as being a breeding site for many seabird species. The islanders retain the right to collect driftwood and guano from the uninhabited island.

Gough Island was exploited by sealers in the late 18th and early 19th centuries, with sealers camping on the island for considerable periods of time. They existed on seabirds and their eggs, fish, wild plants, and cultivated potatoes, which are no longer present on the island. Whaling occurred in the waters of Gough Island between 1830 and 1870, though the island remained uninhabited. The island has been visited by scientists periodically since 1811, and meteorologists have inhabited the island since 1956. The meteorological station, with its associated facilities and helicopter landing site, is the only man made structure on the island. Land birds on Gough Island have been protected under the Tristan da Cunha Wildlife Protection Ordinance since 1950; the Tristan da Cunha Conservation Ordinance of 1976 then declared the island and its waters a wildlife reserve. In 1995, Gough Island and its territorial waters to three nautical miles were inscribed on the World Heritage List, only the third natural British site to be included. Justification for the listing included the fact that Gough is the largest scarcely modified cool temperate island ecosystem in the South Atlantic Ocean; for its huge fur seal and rockhopper penguin populations, endemic landbirds, plants and invertebrates; and its importance as a seabird colony (Swales 1996).

Types and Severity of ThreatsThe isolation of Tristan da Cunha and Gough Islands does not prevent the risk of over-exploitation, which remains a problem with fishing and livestock grazing. The Tristan Islanders have made concerted efforts to prevent degradation and have seen significant success. For example, Phylica arborea trees, which provide an important habitat and food source of for the endemic grosbeak bunting (Nesospiza wilkinsi dunnei), are recovering on cliffs above Edinburgh since a ban on cutting was passed (Swales 1996). Each family on Tristan is now allowed only two cows and seven sheep to reduce the impact of grazing. Some threats, however, are always present, such as the introduction of exotic species onto the islands. The introduced house mouse (Mus musculus) is widespread and abundant on Gough Island (WCMC 1994). Goats and sheep were once introduced to Gough, but are no longer present. All supplies to the meteorological station are carefully checked; in 1990, for example, 10 live snail species were found in an imported cauliflower, along with aphids, caterpillars and mites, and all had to be removed from the island (WCMC 1994). Alien plants on Gough Island primarily originate from seeds found in bird droppings (WCMC 1994). New Zealand Flax, once used for traditional thatched houses on Tristan, had proved to be a problem. It is crucial to keep all of the islands free from exotic mammals, as the important avifauna would be very susceptible to predation from rats or cats.

Tourism is an increasing threat to the environment in all corners of the globe, and Tristan Island’s economy is already partially supported by the sale of postage stamps and handicrafts to non-islanders. The extreme isolation of these islands, the steep and hazardous terrain, and the unpredictability of the weather will likely deter any large-scale tourism.

In 1991, a study was conducted on the effects of the commercial rock-lobster (Jasus tristani) fishery around Tristan, and the most serious impact was found to be the large number of seabirds killed when dazzled by ship's lights at night (Ryan 1991). This affects thousands of individuals of eight or more species annually. Rockhopper penguins (Eudyptes chrysocome) were at one time killed for baiting lobster traps, but this has stopped (Ryan 1991). Unlicensed fishing and illegal use of drift nets occurs to some extent within the Gough reserve (WCMC 1994). Pollution from the meteorological station is carefully controlled, and legislation controls foreign pollutants from vessels passing through Gough’s territorial waters (WCMC 1994).

Justification of Ecoregion DelineationThe Tristan da Cunha and Gough archipelagos are oceanic islands that are isolated to the extent that resident taxa have undergone remarkable speciation. Many floral species are restricted to these two island groups, and three endemic bird genera are present. Each are designated as Endemic Bird Areas of the world by Stattersfield et al. (1998).